As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a fin field effect transistor (FinFET). A typical FinFET is fabricated with a thin vertical “fin” (or fin structure) extending from a substrate formed by, for example, etching away a portion of a silicon layer of the substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over (e.g., wrapping around) the fin. Having a gate on multiple sides of the channel reduces short channel effect and allows for higher current flow.
Designing a FinFET is not without its own challenges. For example, while it is desirable to have a low threshold voltage to increase the on-state current (Ion), having a low threshold voltage can lead to high leakage current, including off-state current (Ioff). Consequently, one of the objectives in designing a FinFET is to achieve a threshold voltage that results in a good Ion/Ioff ratio. The threshold voltage in a FinFET is not constant throughout the height of the fin. It is affected by several factors introduced by the design itself or the fabrication processes. For example, the gate of the FinFET is in contact with three surfaces at the top of the fin while only in contact with two surfaces for the rest of the fin. In addition, the fin may not have uniform shape and width along its height. Furthermore, the distribution of anti-punch through dopants and the source/drain features also affect the threshold voltage. As a result, a non-uniform threshold voltage distribution is usually observed. Often portions of the fin with lower-than-expected threshold voltages suffer from high off-state current density while portions of the fin with higher-than-expected threshold voltage suffer from low on-state current density.
Therefore, although existing FinFET devices have been generally adequate for their intended purposes, they have not been satisfactory in every aspect.